Recently, magnetic recording-reproducing systems employing external media (such as magnetic tapes, particularly, back-up tapes) for computer data have been desired to record the data in a high density. Therefore, it becomes common to use recording signals having a shorter wavelength and recording tracks having a narrower width. Further, in order to rapidly process a great number of data, the processing speed for recording or reproducing the data is accelerated. On the other hand, for attaining a large storage capacity, thinner magnetic tapes such as those having a total thickness in the range of from 3 to 30 μm are widely used. Accordingly, it is desired for the thin magnetic tapes to have high running durability. In the system for high density recording, even a small distortion of the tape often prevents the magnetic tape from smooth running, and further gives an adverse influence to contact between the magnetic tape and a magnetic head so as to impair the output. For this reason, it is also desired for the thin magnetic tape to be resistant to distortion and deformation.
In general, a magnetic tape comprises a flexible support web (e.g., plastic support web) and a magnetic layer provided thereon. For attaining the high density recording, a non-magnetic layer and a thin magnetic layer may be superposed in order on the support. The magnetic layer preferably has a smoother surface to show higher output signals, but the smooth surface often causes troubles in the procedure of winding or running. To avoid the troubles, a back-coat layer is generally provided on the support surface opposite to the magnetic layer. Japanese Patent Provisional Publications No. 5(1993)-217146 and No. 6(1994)-215350, for example, disclose a magnetic tape comprising a magnetic layer on one surface of the support and a back-coat layer on the other surface. Further, Japanese Patent Provisional Publication No. 5(1993)-182178, for example, discloses a magnetic tape comprising a non-magnetic layer and a thin magnetic layer superposed in order on one surface of the support, and a back-coat layer on the other surface.
Since a thin magnetic tape has relatively weak rigidity as compared with a thick one, it is often curled in its longitudinal direction and/or width direction by receiving stress of stretching, compressing or drying in the production process. The curl in the longitudinal direction (which is generally called “coiling”) is often produced by the presence of voids in the magnetic layer. In more detail, the voids are compressed in the calendering process, and then expand to curl the magnetic tape giving a convex surface on the side of the magnetic layer. On the other hand, the curl in the width direction (which is generally called “cupping”) is often produced by residual stress having been applied to the binder of the back-coat layer. When the binder shrinks, the shrunken binder has the residual stress, which thereafter generates a convex surface on the side of the magnetic layer.
The present inventors studied on the curls of a thin magnetic tape and their influence upon electromagnetic characteristics and handling of the magnetic tape, and finally found the following facts.
In the recently developed magnetic recording systems employing thin magnetic tapes, the magnetic tapes are run under a low tension. Accordingly, the curls of the magnetic tape sometimes cause undesirable effects. The curl in the longitudinal direction of the magnetic tape reduces the contact between the running magnetic tape and the magnetic head to impair the electromagnetic output. Further, the curl has an adverse influence on the handling in the process of producing the magnetic tape. Particularly in the step for slitting, the curl often distorts a slit magnetic tape and makes it very difficult to smoothly wind up the magnetic tape. The curl in the width direction sometimes also causes winding troubles, and further at the edge of the magnetic tape the contact with the magnetic head is reduced to impair the electromagnetic output. Further, when the magnetic tape has a curl in the width direction, it becomes difficult for guide apparatuses to give good control on the magnetic tape running so that the tape may move vertically while running so as to cause off-tracks. Particularly, when nitrocellulose (which is a relatively hard resin) is used as a binder in the back-coat layer, the curl in the width direction is sometimes produced and accordingly the electromagnetic output is sometimes impaired.
Japanese Patent Provisional Publication No. 5(1993)-307731 discloses a magnetic tape improved in the curl in the width direction. In the disclosed magnetic tape, the degree of curl in the width direction (i.e., amount of convex curvature on the magnetic layer side) is adjusted within 0.3 to 0.7 mm per ½ inch width (which corresponds to a radius of curvature of 56 to 29 mm). According to the description of the publication, that magnetic tape (having a thin thickness) neither impairs the output nor shows drop-out, and hence exhibits good electromagnetic characteristics. Further, the publication tells that the magnetic tape has enough running durability against distortion or damage. Thus, the publication refers to the problems caused by the curl in the width direction, but is silent with respect to problems caused by the curl in the longitudinal direction.
It is an object of the present invention to provide a magnetic tape suitably employable in the magnetic recording-reproducing systems for storing the data (particularly, computer data) in a high density. It is another object of the invention to give a magnetic tape showing excellent electromagnetic characteristics, good handling property, and high running durability.